The 3rd Generation Partnership Project (3GPP), acting as an important organization in mobile communication field, greatly pushes the standardization of Third Generation (3G) techniques and has proposed a series of standards including Wide Code Division Multiple Access (WCDMA), High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and so on.
Along with the development of communication techniques towards mobility and broadband, power saving problem of User Equipment (UE) has attracted widespread attention. Many communication standard organizations have considered the power saving problem, when making relevant standards. In particular, access networks of future mobile communication systems will be based on Internet Protocol (IP) technique for data transmission. The burst of IP packets and the share of transmission channels among users make the data arrived at the UE in a discontinuous manner. Therefore, it becomes more important to save battery power of the UE.
In the mobile communication system, information exchange between the UE and an access network relies on power supply of both sides. The UE can be a cell phone, laptop and Personal Digital Assistant (PDA) and the like which runs on battery power. Therefore, to lower power consumption and prolong standby and service time of the UE become a key problem for designing the mobile communication system. In order to lower the power consumption of the UE, Discontinuous Reception (DRX) mode is employed in 3GPP standard. In particular, in the DRX mode, the UE monitors a channel and receives downlink services in a pre-defined time interval negotiated with the access network, thereby reducing unnecessary time for monitoring the channel and reducing power consumption of the UE.
In operation, when the UE is connected to a network, the UE and the network negotiate the connection details. In some systems, the UE will enter a “sleep” state and wake periodically to “listen” for paging messages, or be in an active state when a paging message can be received. When sleep states are used, part of the negotiation of the UE with the network may be to determine and set a sleep period, or a period of in-activity. Thus, the network knows when the UE will be expecting paging signals, and can transmit paging signals during a period of time when the UE will be listening for such paging signals.
In current systems, the sleep period negotiated between the UE and the wireless network is fixed. The sleep period may be set according to a profile set up on the UE, and may be different for different types of UE and so on. However, generally, once the sleep period is set by the network, the period is fixed. Also, a sleep period that is too long may result in unnecessary or even unacceptable levels of delay in receiving a paging signal. On the other hand, a sleep period that is too short may significantly reduce the operating life of the battery in the UE (i.e., by depleting battery power by waking up too frequently).
With huge data and longer battery life requirements, efficient power utilization at the UE is critical in current generation wireless networks like Long Term Evolution (LTE) and LTE-Advanced (LTE-A). LTE also supports DRX to allow the UE to save battery power consumption by exploiting the bursty nature of packet arrival, while maintaining minimal packet-delay. The UE can be in two different states, RRC_CONNECTED and RRC_IDL. In the former state, there is a connection established between the UE and the Radio Resource Controller (RRC) and data transfer between UE and the evolved NodeB (eNB) can take place. If there is no data traffic for longer than a specified time, then DRX can be configured by the RRC where the UE is in RRC_CONNECTED state with multiple background applications running.
The UE with DRX functionality enabled monitors the Physical Downlink Control Channel (PDCCH) discontinuously. During each DRX cycle of length Tp, the UE monitors the PDCCH for a duration Ton (on-duration), and then enables the power-down mode for Toff (off-duration), thereby reducing the battery power consumption. Further, with the applications generating uplink (UL) and receiving downlink (DL) packets according to random distributions. With DRX, packets arriving during Toff experience delay in reception or transmission. A higher value of Toff results in lower power consumption, but it increases the delay experienced and therefore there exists a trade-off.